One of the greatest dangers at sea is the risk of collision. This danger is magnified considerably in coastal or inland waterways and ports where shipping is heavy and small boating is popular. Visual watches are limited by weather conditions and available personnel and, while helpful, are generally inadequate. While large ships and many smaller vessels are equipped with radar, this does not assure that the bridge or other personnel are monitoring their equipment. Further, many small vessels do not reflect radar signals adequately, and weak reflected signals may be easily overlooked on a radar screen. Heavy weather and rough seas add to the "clutter" and "noise" appearing on a radar screen, thereby increasing the difficulty of monitoring small boat activity, and may in some circumstances obliterate a signal reflected from a small boat.
Directional finding systems for locating the source of a transmission are well known. Numerous direction finding systems of the prior art employ a circular or cylindrical array of element antennas. Some of the prior systems are suitable for radio direction finding in a range of source directions that is generally broadside to the antenna array, i.e., in the general direction of an axis which is perpendicular to a plane of the circular array of element antennas. Other circularly arrayed direction finding systems of the prior art are suitable for direction finding in and near the plane of the circle of the array. For convenience, directions measured in the plane of the array will be referred to hereinafter as azimuth directions, although no limitation is intended thereby with regard to the orientation of the antenna systems relative to other objects such as a supporting vehicle or the earth. Most azimuth direction finding systems of the prior art have azimuthally directional antenna radiation patterns. For example, a single steerable beam of sensitivity, occupying generally a small sector of the complete azimuthal circle, is provided, which continually revolves through 360.degree. azimuth to locate the azimuth direction from which a target radio wave is coming. In such prior art systems, every direction of azimuth is examined once during each revolution of the steerable beam, and each direction is examined for only a fraction of the period of revolution of the steerable beam. Such systems require complex electronic circuitry and elaborate antenna mechanisms making them generally expensive and unsuitable for small boat applications.